Active studies have been performed in recent years for the development of organic thin film light emitting devices that contain electrodes and an organic fluorescent substance located between them in which electrons injected from the cathode and holes injected from the anode are recombined to emit light. These light emitting devices have attracted attention particularly because of their features such as a thin body, high luminance light emission under a low driving voltage, and multi-color light emission realized by specific type of fluorescent materials.
Since C. W. Tang et. al at Kodak found that an organic thin film device emits light with high luminance, various studies have been carried out to provide practical devices, resulting in the application of organic thin film devices to a variety of instruments such as main displays of portable telephones. However, there still remain many technical problems and one of the major issues is to provide a device that realizes both a high efficiency and a long life.
The required driving voltage for a device depends largely on the carrier transport material used to transport carriers, i.e. holes and electrons, to the light emitting layer. Of these, known materials that can transport holes (hole transport materials) include substances containing an amine backbone (see, for example, Patent documents 1 to 2), a carbazole backbone (see, for example, Patent document 3), or an indolocarbazole backbone (see, for example, Patent document 4). Substances containing an amine backbone are useful because they show high hole transport performance, but they suffers a significant deterioration in triplet energy as a result of an excessively long conjugation. Therefore, they cannot work effectively in confining triplet excitons particularly from a phosphorescene emitting layer, resulting in a low luminous efficiency. In addition, there also remain problems concerning the heat resistance of materials and durability of devices. Furthermore, it is known that substances containing a carbazole backbone or indolocarbazole backbone are high in the triplet level and it has been proposed to use them as material for phosphorescene emitting layers or material for confining triplet excitons from a phosphorescene emitting layer (see, for example, Patent document 5). Accordingly, they are particularly suitable for devices that require high triplet energy, such as green phosphorescent devices. However, substances high in triplet energy generally tend to be high in singlet energy and inevitably high in ionization potential. Therefore, devices that do not require a very large triplet energy, such as fluorescent devices and red phosphorescent devices, will need an increased driving voltage, leading to a decrease in luminous efficiency and a deterioration in device durability.